1. Field of Invention
The present invention relates to an imaging device that shoots images using an imaging element and in which exposure is controlled by controlling an accumulation time of light received by the imaging element, and that is equipped with a shutter that shades the imaging element.
2. Description of Related Art
In an electronic camera that uses a solid-state imaging element, an electronic shutter can be used to perform exposure control by controlling the accumulation time of photoelectric charge of a photosensitive part of the imaging element. Methods of implementing this kind of electronic shutter vary depending upon the imaging element. For example, in the interline (IT) type CCD (charge coupled device) as shown in FIG. 11A, a photoelectric charge is accumulated in the photodiodes of the photosensitive part A during a specified interval and the signal charge is then transferred to a vertical transferring part B. The electronic shutter is further implemented by transferring the signal charge from the vertical transferring part B to an output circuit D via a horizontal transferring part C.
At this time, when the accumulated charge is transferred from the photosensitive part A to the vertical transferring part B, it is known that there is a problem of generating smear (a phenomenon in which a vertical white line can be seen when the light intensity is strong) by leakage of charge or the like from the photosensitive part A.
Additionally, in the frame transferring (FT) type CCD as shown in FIG. 11B, the electronic shutter is implemented by resetting the pixels, that is, by setting the accumulated charge amount of the pixels to zero, and then, after the elapse of a specified exposure time, transferring the charge of the photosensitive part A to the accumulation part E at high speed and further, transferring it to a horizontal transferring part C and then to output circuit D.
However, in the case of the above-mentioned FT-CCD, the generation of smear cannot be controlled because the light can be received (by the photosensitive part A) even during the transfer of charge to the accumulation part E. In particular, the ratio between the photoelectric charge time and the signal transfer time to the accumulating part E for one level of the CCD is equal to the ratio between the signals and the smear amount. Thus, there is a problem in that a high speed electronic shutter cannot be easily realized that sufficiently controls smear.
As a method of solving the problem of smear, it is known to provide the necessary accumulation part (undepicted) needed for the vertical transferring part B of the IT-CCD and to transfer the signal charge from the vertical transferring part B to the accumulation part at high speed. This reduces as much as possible the time that the signal charge stays in the vertical transferring part B, which is adjacent to the photosensitive part A, which is where the smear is generated. However, in such a structure, there is a problem of increasing the chip area. That is, due to the provision of the accumulation part, the overall size of the CCD chip is increased. An additional problem is that when the number of pixels increases, it becomes difficult to transfer signal charge to the accumulation part in a short period of time because the number of the vertical transferring steps increases, and the consumed power becomes large.
Meanwhile, an electronic shutter can be implemented using a scanning method of the so-called XY address type with either an amplifying type image element or with a MOS type imaging element. The electronic shutter can be implemented in such devices by, first, reset-scanning the pixels (performing scanning that temporarily sets the accumulated charge amount of the pixels to zero) per pixel or per line, and after a specified time for one pixel or line, respectively, passes, performing the scanning for reading of the signal (i.e, performing reading scanning).
FIG. 12 shows an example of a MOS type imaging element of the XY address type disclosed in Japanese Laid-Open Pat. Publication No. 55-145481 (corresponding to U.S. Pat. No. 4,335,405). In the image elements of FIG. 12, two circuits, a signal reading scanning circuit (labeled VSRSignal), and a reset scanning circuit (labeled VSRReset), are provided as vertical scanning circuits (VSR). An electronic shutter is implemented by first performing reset scanning in the reset scanning circuit, and then after a specified time, performing the reading scanning in the signal reading scanning circuit.
In the amplifying type imaging element of the XY address type disclosed in Japanese Laid-Open Pat. Publication No. 5-227489 (corresponding to U.S. Pat. No. 5,410,348), the electronic shutter is implemented by dividing the reset scanning and the signal reading scanning by time in one vertical scanning circuit.
However, these kinds of electronic shutters have a problem in that the exposure time in the upper and lower parts is shifted for only the time required for scanning of the screen. Therefore, the scanning of the signal output cannot be performed at high speed, especially when there are many pixels, which limits their use as an electronic shutter.
Furthermore, in an electronic camera that shoots a still image, exposure time control can be performed by opening a mechanical shutter that optically shades the imaging elements from incident light for a required time. However, ordinary moving image photography cannot be performed when the shutter is closed, and thus a separate viewfinder optical system is needed in order to monitor the object prior to shooting.
U.S. Pat. No. 4,535,363 discloses a camera in which a still image is shot and a moving image is shot by opening the shutter before and closing the shutter just after the reading period of the image signal. When that technique is implemented when an IT-CCD imaging element is used, the charges stored in the IT-CCD pixels are reset in a batch, reading is also performed in a batch, and the charges are read into the vertical transferring part, which is shaded from the photosensitive part of the IT-CCD. Therefore, by shading the IT-CCD by a simple shutter structural body such as a rotating shutter, just during the time there is charge in the vertical transferring part of the IT-CCD, it is possible to prevent smear generation. Meanwhile, in order to perform the same technique in an FT-CCD, it is necessary to simultaneously shade all the pixels, and a special shutter such as a diaphragm that doubles as a lens shutter must be used. However, both of these techniques have not yet solved the problem of smear during shooting of the moving image.
Meanwhile, in the imaging elements of the XY address type, the reading point vanes depending upon each pixel or line. Thus, an effective means that closes the shutter to achieve shading only during reading is not known. In particular, in the amplifying type imaging elements of the XY address type, even though an electronic shutter is implemented, there is an advantage in that there is no smear generation so that the smear does not occur during the shooting of the moving image. However, an imaging device which effectively uses it is not known.
Therefore, an object of the present invention is to provide an imaging device that shoots a moving image and in which smear does not occur and that further shoots a still image with a sufficiently accurate shutter operation.
According to a first aspect of the present invention, an imaging device includes an imaging element that accumulates charge from received light. A shutter moves so as to shade the imaging element. A controller controls the device to perform charge accumulation start scanning of the imaging element at a timing and/or speed based on the travel speed of the shutter, and then to perform reading scanning of the charges accumulated in the imaging element.
According to this structure, it is possible to shoot a still image in addition to a moving image. When the shutter is caused to travel in order to shoot a still image, it is possible to shoot the still image with a uniform exposure time in all areas of the imaging element.
The controller also can adjust the exposure time by controlling the scanning start time of the charge accumulation start scanning and the travel start time of the shutter when the still image is shot. According to this structure, it is possible to arbitrarily adjust the exposure time during the shooting of the still image.
The exposure time also can be set in response to the output level of the charges read by the scanning of the reading scanning without causing the shutter to travel after the scanning of the charge accumulation start scanning prior to shooting the still image. According to this structure, photometry of the object can be performed based upon the output level of the charges read during the shooting of the moving image and it is possible to set the exposure time during the shooting of the still image based upon that photometry.
As another option, the controller can control the characteristic of the scanning speed of the charge accumulation start scanning so as to match the changing characteristic of the travel speed of the shutter. According to this structure, it is possible to ensure a certain exposure time during the shooting of the still image even though the travel speed of the shutter is not constant.
As another option, the controller can control the scanning speed of the reading scanning so as to be longer than the scanning of the charge accumulation start scanning. According to this structure, it is possible to accurately perform the reading scanning.
As another option, the controller can control the scanning start time of the reading scanning so as to sequentially read the accumulated charges from the area of the imaging element by which the shutter has traveled and shaded. According to this structure, it is possible to start the reading scanning before the traveling of the shutter is completed.
As another option, the controller can generate a scanning start signal that instructs the scanning start of the charge accumulation start scanning and a travel start signal that instructs the scanning start of the shutter. Additionally, the generation period of the scanning start signal and of the travel start signal can be set based upon the exposure time and the delay time between the time when the travel start signal is generated and the time when the shutter actually begins to travel. According to this structure, it is possible to accurately control the exposure time during the shooting of the still image.
As another option, the scanning line of the imaging element scanned by the charge accumulation start scanning and by the reading scanning and the scanning line of the imaging element shaded by the travel of the shutter are substantially parallel. According to this structure, it is possible to ensure a uniform exposure time in the entire area of the imaging element.
According to another option, an image processor processes the charges read by the reading scanning as an image and a display displays the image processed by the image processor. In particular, the display displays the image obtained by alternately repeating the scanning of the charge accumulation start scanning and the reading scanning as a moving image without causing the shutter to travel. According to this structure, it is possible to monitor the moving image.
As another option, the controller can adjust the exposure time by causing the charge accumulation start scanning and the reading scanning to scan at the same speed and by controlling the scanning start period thereof. According to this structure, it is possible to arbitrarily adjust the exposure time during the shooting of the moving image.
As another option, the shutter can be a focal plane type shutter with at least one shutter curtain. According to this structure, it is possible to perform a shading of the imaging element that corresponds to the charge accumulation start scanning.
As another option, the imaging element can be an XY address type imaging element. According to this structure, it is possible to perform a charge accumulation start scanning that corresponds to the traveling of the shutter.
As another option, the imaging element can be an amplifying type imaging element. According to this structure, it is possible to shoot the moving image with less smear.